1. Field of the Invention
This invention relates to an objective lens driving device which is applicable to an information reading-out apparatus for projecting a reading-out light spot onto an information track which has spirally or concentrically been recorded on a record medium, for example, and reading out such information and which can drive the objective lens in a direction perpendicular to the optical axis of the objective lens and to the information track for the purpose of correcting the relative deviation in position between the information track and the reading-out light spot projected through the objective lens onto the information track.
2. Description of the Prior Art
The above mentioned information reading-out apparatus has heretofore been well known. The record medium including the information track recorded thereon is called a video disc. On the information track of the video disc is recorded an encoded video signal or voice signal as optical information by such means as optical transmitting, reflecting and phase properties. The video disc is rotated at high speed and a laser beam emitted from a laser light source is focussed through an objective lens as a reading-out light spot. The reflected light or transmitted light modulated by the information track is detected and read out. One of the features of such a record medium is a very high density of recorded information and hence the width of each information track is very narrow and the space between successive tracks is also very narrow. As a result, the diameter of the reading-out light spot should be correspondingly very small. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, a mutual positional error between the reading-out light spot and the information track, i.e. a tracking error, should be reduced to as little as possible. It has been proposed to effect a so-called tracking control in which mutual displacement of the reading-out light spot and the information track is photoelectrically detected to produce a tracking error signal and the reading-out light spot is displaced in a direction perpendicular to the information track in response to the detected tracking error signal. In such a tracking mechanism heretofore proposed, use is made of an oscillating mirror inserted in an optical path from a laser light source to an objective lens, and the mirror is rotated in response to the detected tracking error signal. However, such a tracking mechanism could not attain a sufficiently high accuracy and is liable to be large in size and expensive.
In order to avoid the above mentioned drawbacks of the known tracking mechanism, it has been further proposed that the objective lens or its holder be supported by a resiliently supporting member comprising springs, e.g. leaf springs, and that the objective lens be moved in a direction perpendicular to the optical axis of the objective lens as well as to the tangential direction of the information track by means of an electromechanical transducer in response to the tracking error signal. As the electromechanical transducer, use may be made of an electromagnet, voice coil, piezoelectric element or the like. In order to obtain a good response property for tracking, the assembly should be made small in size and light in weight.
In an actual reading-out apparatus, in addition to the tracking error a so-called focussing error is produced in which the light spot could not be correctly focussed on the information track. For correcting the focussing error, provision must be made of for a focussing mechanism for displacing the objective lens in the direction of its optical axis. When the tracking mechanism is mounted on the focussing mechanism, the tracking mechanism should be smaller in size and lighter in weight for attaining an accurate focussing correction.
In a tracking mechanism which makes use of an electromagnet, a sufficient force necessary for moving the objective lens can be produced and the mechanism can be made small in size and light in weight in a relatively easy manner. However, an accurate tracking correction could not be effected, because the relation between an amount of electric current flowing through a coil of the electromagnet and an amount of displacement of the objective lens is not linear. In the tracking mechanism comprising the voice coil, miniaturization could hardly be attained. In addition, the tracking mechanism which makes use of the piezoelectric elements could not produce a sufficiently large driving force for driving the objective lens.